6-O-methylerythromycin A (clarithromycin), shown below, is a potent macrolide antibiotic disclosed in U.S. Pat. No. 4,331,803. ##STR1##
In general, the process for making clarithromycin can be thought of as a four-step procedure beginning with erythromycin A as the starting material:
Step 1: optionally convert the 9-oxo group to an oxime;
Step 2: protect the 2' and 4" hydroxyl groups;
Step 3: methylate the 6-hydroxyl group;
Step 4: deprotect at the 2', 4" and 9-positions.
A variety of means for preparing 6-O-methylerythromycin A have been described. 6-O-methylerythromycin A can be prepared by methylating a 2'-O-3'-N-dibenzyloxycarbonyl-des-N-methyl derivative of erythromycin A (U.S. Pat. No. 4,331,803). 6-O-methylerythromycin A can also be made from 9-oxime erythromycin A derivatives (See, e.g., U.S. Pat. Nos. 5,274,085; 4,680,386; 4,668,776; 4,670,549 and 4,672,109 and European Patent Application 0260938 A2).
In those reports relating to 9-oxime erythromycin A derivatives, the oxime is protected during methylation with a 2-alkenyl group (U.S. Pat. Nos. 4,670,549 and 4,668,776), a benzyl or substituted benzyl group (U.S. Pat. Nos. 4,680,386, and 4,670,549) or a moiety selected from the group consisting of lower alkyl, substituted alkyl, lower alkenyl, aryl substituted methyl, substituted oxalkyl, and substituted thiomethyl (U.S. Pat. No. 4,672,109).
There are drawbacks to the existing methods for producing 6-O-methylerythromycin A. By way of example, failure to protect the 2'-OH group leads to undesired methylation of that group. Existing methods for protecting the 2'-OH group are unsatisfactory because those methods also require protection of the 3'-nitrogen. U.S. Pat. No. 4,680,386 discloses protection of the 2'-OH group with a benzyloxy carbonyl moiety. Under such circumstances, however, the 3'-nitrogen also undergoes N-demethylation followed by N-benzyloxy carbonyl formation. This 3'-N-benzyloxy carbonyl group must be deprotected following 6-O-methylation. The 3'-dimethylamino group is regenerated following 6-O-methylation by N-methylation. U.S. Pat. No. 4,670,549 discloses protection of the 2'-OH group as a benzyl or like substituent. Under these circumstances, the 3'-nitrogen group must also be protected as a quaternary salt. This quaternary salt must be removed following 6-O-methylation to regenerate the 3'-dimethyl amino group. By way of further example, the use of benzyloxycarbonyl groups for protection of the 2'-hydroxy group (U.S. Pat. No. 4,331,803) requires large amounts of benzyl chloroformate, which is severely irritating and toxic.
There continues to be a need to provide a rapid, efficient method of producing 6-O-alkylerythromycin compounds that uses mild, neutral synthetic conditions. In particular, it is desirable to provide a process which does not require protection of the 2'-hydroxy group.